Characterization of intermetallic phases and oxides formed in annealed Ni/Al multilayer structures

Two different multilayer structures composed of ten alternating Ni and Al thin films were sputter deposited on Si (111) substrates. These multilayers with individual Ni and Al thin film thicknesses of about 25 nm and 38 nm and of 25 nm and 13 nm, respectively, have the average compositions of Ni_0.50Al_0.50 and Ni_0.75Al_0.25. The samples were heat treated in a differential scanning calorimeter instrument with a constant heating rate of 40 °C min ⁻¹ in Ar from room temperature to 550 °C. The compositions of as-deposited and heat-treated samples were studied with high-resolution Auger electron spectroscopy (AES) rotational depth profiling. X-ray photoelectron spectroscopy (XPS) analyses show an excess of Ni in both annealed samples. X-ray diffraction measurements of annealed multilayers show the formation of Ni₂Al₃ and NiAl₃ phases in the Ni_0.50Al_0.50 sample and the presence of Ni₃Al and Ni A1₃ phases with some excess of Ni in the Ni_0.75Al_0.75 sample. AES and XPS investigations of the reacted layers after 15 min annealing in air at 500 °C disclose considerably different surface oxide thin films: on the Ni_0.50Al_0.50 layer the oxide thin film consists of Al₂O₃ with a small amount of NiO, whereas that on the top of the Ni_0.75Al_0.25 layer is thicker and consists of NiO on top and some Al₂O₃ below.     

Comparative study of martensitic transformation behavior of NiAlMn and NiAlMnFe high temperature shape memory alloys

A comparative study of microstructure and martensitic transformation (MT) behavior of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ high temperature shape memory alloys (SMAs) has been performed by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS), optical microscopy, and micro-hardness testing. The MT temperature (MTT) of Ni₅₉Al₁₁Mn₃₀ alloy is higher than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy, and both alloys’ MTT increases with increasing annealing temperature. The temperature hysteresis and hardness of Ni₅₉Al₁₁Mn₃₀ alloy are smaller than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy. The MT behavior of Ni₆₀Al₁₉Mn₁₆Fe₅ is sensitive to aging temperature and its MTT and hysteresis decrease with increasing aging temperature. However, the MT behavior of Ni₅₉Al₁₁Mn₃₀ alloy is not sensitive to aging temperature. The MT stabilization effects appear in both alloys during thermal cycles. This stabilization effect vanishes from the second thermal cycle. The quenched microstructure of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ alloys is M plus gamma phase, in which the volume fraction of gamma phase is about 40 and 20%, respectively, and the microhardness of M is higher than that of gamma phase. No aging effects were found in both alloys after aging at 400 °C.     

Characterization of mechanically alloyed ternary Fe–Ti–Al powders

The effects of Ti-substitution for Fe in the Fe₃Al system on the mechanical alloying process were investigated. For this purpose, blended elemental powders with the following nominal compositions (at.%): Fe₇₅Al₂₅, Fe₇₀Ti₅Al₂₅, Fe₆₅Ti₁₀Al₂₅, Fe₆₀Ti₁₅Al₂₅, were mechanically alloyed in a high energy attritor-type ball milling system for up to 100 h. The structural evolution in these powders was characterized by scanning electron microscopy, differential thermal analysis and X-ray diffraction techniques. It was found that elemental powders were progressively transformed into nanocrystalline solid solutions during mechanical alloying. The addition of Ti in the powders shortened the milling time for solid solution formation. With increasing Ti content, the grain size of the solid solutions decreased, but the lattice parameter increased. Upon heating, the milled powders were transformed into ordered (Fe,Ti)₃Al intermetallic compounds in an extended range of temperature (from 350 to 500°C). Ti addition enhanced the occurrence of DO₃ ordering in heated powders.     

FABRICATION OF A Ni3Al/Al2O3UNIDIRECTIONAL COMPOSITE BY PRESSURE CASTING

A gas pressure liquid metal infiltration technique for producing fiber reinforced Ni₃Al/Al₂O₃ and Ni₃Al/Al₂O₃ composites has been described. Composite bars of 9.5 mm diameter, 150 mm long were produced. It was found necessary to add 0.5 at.%Ti to both Ni₃Al and NiAl alloys to obtain adequate wetting. There was a strong tendency for liquid metal infiltration along one side of the fibers causing a shift of the fibers to the opposite side. No interface reaction zone was observed at optical magnifications up to 1000X. A composite of Ni₃Al containing 33 volume percent of FP fibers fractured at 890 MPa which is approximately 3 times the yield strength of the matrix IC-50(21.73A1-0.34Zr-0.IB-Balance Ni at.%) + 0.5 at. %Ti.     

Grain growth mechanisms in a spray-formed Ni3Al/Al2O3 composite in the presence of a liquid phase

Grain growth in the two-phase (liquid + solid) region of Ni₃Al reinforced with 0.8 vol.% Al₂O₃ participates synthesized by a spray atomization and co-injection technique was investigated. The grain growth of the as-sprayed and hot isostatically pressed (HIPed) materials in the two-phase region was found to be consistent with cube law kinetics, i.e., grain growth exponent was approximately 3. The activation energy for grain growth for the as-sprayed material was determined to be 308 ± 19 kJ mol⁻¹ while that of the HIPed material was calculated to be 327 ± 23 kJ mol⁻¹. The activation energy for grain growth was not a function of the amount of liquid phase or the composition of the liquid. Furthermore, the activation energy for grain growth was higher than that for diffusion through the liquid phase, suggesting that the mechanism for grain growth of the as-sprayed and HIPed Ni₃Al composite in the two-phase region was controlled by an interface reaction. The role of the second-phase Al₂O₃ particles on grain growth for the as-sprayed and HIPed Ni₃Al materials was not significant.     

Growth Mechanisms and Hot Corrosion Resistance of Palladium Modified Aluminide Coatings on Superalloys

Recently palladium has generated interest as a precoating for aluminizing superalloys used in gas turbine blades. Little work has however been done on the mechanisms of formation of palladium modified aluminide coatings, and also on the constitution of the ternary Ni-Pd-Al system. The present work was initiated to fill this gap. It was first devoted to a study of the ternary phase diagram, with synthesis of alloys in order to evaluate the stability domains of the different known phases: Ni₃ Al, NiAl, Ni₂Al₃, Pd₂Al, PdAl, Pd₂Al₂. Annealing of diffusion couples made with different alloys of the two binary Pd-Al and Ni-Al systems enabled to complete this study. Formation mechanisms of the coatings were also determined, with aluminization of binary PdNi alloys in different conditions (low and high aluminum activity). Resistance to hot corrosion of palladium modified aluminide coatings was evaluated with burner rig tests, and was compared to that of simple and platinum modified aluminide coatings.     

碳钢坩埚表面渗铝复合涂层

以碳钢板为基板材料, 通过表面渗铝和高温化学反应在其表面形成复合保护涂层。研究了反应层厚度与反应温度、时间之间的关系, 并用光学显微镜、XRD对涂层形貌、相组成进行了表征。实验结果表明: 反应产物层厚度随反应温度、时间的增加而增加; 复合涂层由过渡层和反应产物层组成, 过渡层组成为Fe₃Al及少量Fe₂Al₅、Fe₁₄Al₈₆、Al₂O₃, 反应产物层组成为TiB₂、MgO和少量的Mg₂TiO₄、Mg₂B₂O₅、Fe₃Al、FeAl、Ti₂B₅。      

Phase transformations in the alloy hastelloy B

The alloy Hastelloy B undergoes phase transformations in the temperature range of 600°–800° C. These phase transformations were studied in some detail by electron microscopy. The essential result of this study was the observed formation of the DO₂₂ structure as an intermediate phase. The DO₂₂ structure is relatively stable in Hastelloy B and its further transformations are easily observable. At 600°C it is transformed to Ni₄ Mo followed by a partial transformation to Ni₃ Mo, whereas at 700°C Ni₃ Mo is formed from DO₂₂ directly.     

Amorphous Ni---N---W film as a diffusion barrier between aluminum and silicon

The influence of nitrogen on the diffusion barrier properties of amorphous Ni---W films was studied. Nitrogen was introduced into the amorphous Ni---W film by co-sputtering nickel and tungsten in a premixed gas mixture of 90% Ar and 10% N₂, resulting in the formation of amorphous Ni₃₀N₂₁W₄₉ film. X-ray analysis indicates a detectable crystallization of the amorphous film after 30 min annealing in vacuum at 600°C, accompanied by the formation of W₂N, but backscattering spectrometry (BS) reveals a reaction with silicon only at about 725°C. The Schottky barrier height of this amorphous film on n-Si is stable for 30 min annealing up to at least 550°C. With an aluminum overlayer, BS indicates that an amorphous Ni₃₀N₂₁W₄₉ film effectively prevents the metallurgical interaction between aluminum and silicon for 30 min up to 600°C. The Schottky barrier height of that contact configuration is also stable up to at least 550°C, suggesting that amorphous Ni---N---W films have attractive features as diffusion barriers.     

Processing and Characterization of Fiber Reinforced Intermetallic Matrix Composites

A series of intermetallic matrix composites reinforced with Al₂O₃ based fibers were fabricated by pressure casting. The Al₂O₃ based fibers used were DuPont's 20 μm diameter Fiber FP and PRD-166 fiber, Mitsui's 10 μm diameter Almax fiber, and Saphikon's 125 μm diameter single crystal Al₂O₃ fiber. The intermetallic matrices employed were alloys based on Ni₃Al, NiAl, Fe₃Al, Ti₃Al+TiAl, and Nb₂Al+NbAl₃. Optical, scanning and transmission electron microscopy were used to investigate the microstructure of the composites and the fibers. Tensile testing was conducted to determine the Weibull mean strength of the fibers in the as-received and heat treated conditions. The effect of fiber gage length on the Weibull mean strength of the PRD-166 and Fiber FP was evaluated. Indentation tests were performed to determine the effect of alloying additions on the fiber/matrix bond strength in shear in Saphikon fiber reinforced Ni₃Al composites.     

Effect of nanocrystalline binder on the microstructure and mechanical properties of ultrafine Ti(CN) cermets

The influence of highly deformed nanocrystalline binder ingredient on the overall microstructure and mechanical properties of ultrafine grade Ti(C_0.5N_0.5) cermet was investigated. Nanocrystalline Ni and Ni-aluminides viz. NiAl and Ni₃Al, synthesized by mechanical milling/alloying, were blended to the cermet powder prior to sintering. The mechanically milled nano-Ni contained a mixture of f.c.c. and hexagonal Ni phases. The cermet with nanocrystalline Ni showed a considerably improved microstructure over cermets with commercial coarse grade Ni and exhibits a high Vickers hardness of 16.1 GPa, along with a good fracture toughness value of 9 MPa m^1/2. However, no significant change in the mechanical properties could be detected in the case of nanocrystalline NiAl and Ni₃Al binder addition. The enhanced properties of the cermets containing nano-Ni were attributed to the finer particle size of the hard phase after sintering along with improved particle size distribution caused by rapid dissolution, which prevents excessive coalescence of the hard particles. On the other hand, the presence of high porosity in the cermets containing nano-NiAl and -Ni₃Al apparently nullified any such enhancement.     

Microstructural characterization of P/M Ni3Al consolidated by HIP

Rapidly solidified powder of Ni₃Al doped with boron was produced by inert gas atomization and consolidated by hot isostatic pressing (HIP). Morphology and microstructure of the powder were studied. From the particle morphology, it could be deduced that the solidification time was similar at least to the time necessary for complete fragmentation of the liquid. The powder showed a two-phase microstructure that was finer the smaller the particle size. The presence of dendrites of NiAl (β) phase was consistent with the diagram proposed by Schramm and not with the traditional diagram of Singleton et al. The microstructure of the material consolidated at 1100°C and 1200°C was studied. A monophasic structure was observed after HIP, and no relevant microstructural differences were seen between the two temperatures used.     

On the Ni5Al3 phase and related phenomena in a NiAlFe alloy

The Ni₅Al₃ Phase and related phenomena have been systematically studied using a Ni-25Al-15Fe alloy, with conventional and in situ optical microscopy and electrical resistance measurements. The present experimental results are comprehensively discussed in comparison with previous reports in order to clarify the complex characteristics of the Ni₅Al₃ phase and its formation mechanism.     

Microstructure and strength of diffusion-bonded joints of TiAl base alloy to steel

Diffusion bonding of TiAl-based alloy to steel was carried out at 850–1100 °C for 1–60 min under a pressure of 5–40 MPa in this paper. The relationship of the bond parameters and tensile strength of the joints was discussed, and the optimum bond parameters were obtained. When products are diffusion-bonded, the optimum bond parameters are as follows: bonding temperature is 930–960 °C, bonding pressure is 20–25 MPa, bonding time is 5–6 min. The maximum tensile strength of the joint is 170–185 MPa. The reaction products and the interface structures of the joints were investigated by scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA) and X-ray diffraction (XRD). Three kinds of reaction products were observed to have formed during the diffusion bonding of TiAl-based alloy to steel, namely Ti₃Al+FeAl+FeAl₂ intermetallic compounds formed close to the TiAl-based alloy. A decarbonised layer formed close to the steel and a face-centered cubic TiC formed in the middle. The interface structure of diffusion-bonded TiAl/steel joints is TiAl/Ti₃Al+FeAl+FeAl₂/TiC/decarbonised layer/steel, and this structure will not change with bond time once it forms. The formation of the intermetallic compounds results in the embrittlement of the joint and poor joint properties. The thickness of each reaction layer increases with bonding time according to a parabolic law. The activation energy Q and the growth velocity K₀ of the reacting layer Ti₃Al+FeAl+FeAl₂+TiC in the diffusion-bonded joints of TiAl base alloy to steel are 203 kJ/mol and 6.07 mm²/s, respectively. Careful control of the growth of the reacting layer Ti₃Al+FeAl+FeAl₂+TiC can influence the final joint strength.     

Formation of intermetallic compounds in the Cr–Al–Si ternary system

The microstructure and composition of binary and ternary intermetallics have been studied in ternary diffusion couples of Cr and an Al–Si eutectic alloy. The ternary intermetallic always formed in the liquid part of the diffusion couple as a dendritic structure. Two intermetallics compounds, CrSi₂ and Cr₅Si₃, of the Cr–Si binary system have been observed. The CrSi₂ intermetallic has a high solubility of up to 20 at.% Al and forms as faceted plates. A number of intermetallics, namely, CrAl₇, Cr₂Al₁₁, CrAl₄, Cr₄Al₉, Cr₅Al₈ and Cr₂Al, of the Cr–Al system have been observed. The solubility of Si varies from as low as 0.8 at.% in Cr₂Al to as high as 9 at.% in Cr₄Al₉. A schematic of the reaction scheme of the Cr–Al–Si system is presented. This has been based on the observed microstructure and composition of phases.     

METAL MATRIX COMPOSITES REINFORCED WITH INTERMETALLIC RIBBONS

The design procedure for low melting point alloy composites reinforced with melt-spun intermetallic ribbons for elevated temperature applications is presented. Long-range ordered Ni₃Al intermetallic is selected as a candidate reinforcement by virtue of its ease of being ductilized by boron and an increase in yield strength with increasing temperature. A certain composition of Ni₃Al alloyed with cobalt, tantalum, hafnium and boron for the maximum reinforcing effect is formulated. The rule of mixtures calculations of elastic modulus and yield strength are performed for Al-Si and Mg-Al matrices. The results of the structural investigation of a model composite system consisting of a 1100 Al matrix with embedded Ni₇₅Al₂₃Zr₁B₁ (at.%) ribbons, obtained by casting, are presented. Long-term stability of the reinforcing ribbons is investigated by annealing of “as-cast” specimens at elevated temperatures up to 100 h.     

Growth of C60 single crystal films on metal substrates

High-quality C₆₀(111) single crystal films have been grown on Ni₃Fe(111), Ni₃Co(111) and Ni₃Fe(110) surfaces using hot-wall diffusion method. X-ray diffraction results show that well-ordered films can be obtained near 150°C on these substrates. The high quality of the films could be attributed to the perfect lattice match between C₆₀ film and substrates, low growth rate of C₆₀, and the suitable substrate temperature.     

Pulsed laser deposition of NiTi shape memory alloy thin films with optimum parameters

A series of experiments was carried out to optimize the pulsed laser deposition parameters for the fabrication of high quality NiTi shape memory alloy thin films. Smooth NiTi shape memory alloy thin films were deposited at high growth rate with optimum deposition parameters based on the analysis of the relationships among the morphology of the target surface and the deposited thin film, the laser energy, the target–substrate distance, the thin film composition and its growth rate. Crystal structures and phase transformation temperatures of the annealed Ni_49.7Ti_50.3 thin film were characterized by using X-ray diffraction and differential scanning calorimetry, respectively. The martensitic transformation temperature of the crystallized Ni_49.7Ti_50.3 thin film is found to be lower than room temperature and 27°C lower than that of the NiTi target material. These results are attributed to the refined grain size of the thin film and its composition, which deviates slightly from Ni₅₀Ti₅₀.     

Processing and Heat Treatment Effects on an Al2O3/Ni3Al Composite

Various processing routes may be used to fabricate Ni₃Al matrix composites. In the present study, vacuum hot pressing and hot extrusion were used to fabricate an Al₂O₃ Composite particulate-reinforced Ni3Al composite. Relatively low extrusion temperatures promoted gamma-phase formation as did extended high temperature anneals. The room-temperature bend strength was found to increase as the gamma-phase content increased. Furthermore, the composite creep rate was affected by the. processing parameters and their effect on the amount of gamma/pgamma-prime interface. The matrix and particulates did not exhibit any reaction after normal processing. However, a minor reaction was noted after annealing near 1000°C, while extensive reaction resulting in Ni-Al spinel formation occurred above 1200°C in air     

CHEMICAL VAPOR DEPOSITION OF TiB2 FOR TiN-TiB2 LAMINAR COMPOSITES

TiB₂ is a material with very interesting properties with respect to erosion and corrosion resistance. Deposition on metallic substrates using TiCI₄, BBr₃ or BCI₃ and H₂ at temperatures around 900° C results in coronation of the substrate, which is most severe when using BBr₃. Therefore, a TiN diffusion barrier is applied. Here we discuss the deposition of TiB₂ using BCI₃ on molybdenum and TiN and compare the results with those of the thermodynamically more favorable reaction with BBr₃. Smooth TiB₂ layers are formed when using BCI₃, with faceting occurring above 900° C. The morphology seems to be independent of the BCI₃/TiCl₄ ratio in the gas phase for values between 0·5 and 4. With an excess of boron in the gas phase - BCI₃/TiCl₄ = 8, depletion occurs already at 800° C. An apparent activation energy of 210 KJ/mol has been determined for a stoichiometric gas phase with BCl₃/TiCI₄ = 2. When the supply of boron is limiting - BCl₃/TiCl₄ = 0·5, the activation energy is 120 KJ/mol.